1. Fields of the Invention
The present invention pertains to arched buildings and roofs. More directly, it relates to the skeletal arched frameworks which must resist the high and variable wind and snow loads such arched structures are exposed to.
Historically, arched structures have been large bridges and buildings for public uses such as auditoriums and sports arenas. These structures have large spans and arch shapes which are close to the funicular shape for the dead load. An open-web steel arch bridge spans 1,652 feet at Bayonne, N.J. (Ref. 1, McGraw-Hill Encyclopedia of Science & Technology, 7th Ed., 1992, pg. 50). A reinforced concrete arched vault spans 262 feet in the Turin Exhibition Hall (Ref. 2, Luigi Nervi, Aesthetics and Technology in Building, Harvard University Press, Cambridge, Mass., 1965). Laminated wood arched structures have spans over 200 feet, with monolithic three-hinged arches (Ref. 1, pg. 51) and over 500 feet as a lamella dome in Tacoma, Wash. (Ref. 3, Wood Handbook: Wood as an Engineering Material, Agriculture Handbook 72, Forest Service, USDA, 1987, pg. 10-4).
This invention relates to open-web arches which may be freely shaped to meet aesthetic and functional ends, independent of building site and resulting live loads. Such arches have non-funicular shapes and are primarily stressed by internal bending moments resulting from structure live loads of wind and snow. They will primarily be used in residential structures having clear spans less than fifty feet.
2. Description of the Related Art
Nine U.S. Patents have been found which disclose compound arched structures composed of two or more arches which are spaced apart in directions radial to their common cross section areal centroid curve and connected by a set of sturctural parts. Four of the older inventions (1889-1944) used wooden arches and four of the later ones (1942-1970) used structural steel arches. One (1969), an arched tower of approximately funicular shape rather than a moment resisting structure frame, used glass reinforced plastic arches, for their electrical and structural properties.
The relevance of these open-web arched structures to the present invention can best be seen by first considering the function of a straight, transversely loaded beam. Assume a horizontal beam is loaded vertically downward in its depth direction. The top half of the beam is compressed and tends to shorten. The lower half tends to lengthen under tensile stress. The middle part of the beam develops shear stress to resist relative longitudinal sliding between the top and bottom beam halves. Shear stress and strain in the longitudinal and depth directions produce tensile and compressive stress and strain in directions having acute angles of plus and minus 45.degree. to the longitudinal beam axis. For this reason, the mid-depth portion of a monolithic beam can be replaced by diagonal bars inclined at or near 45.degree. to the beam axis. Such open-web beams are common in steel joist and wood trusses for both floors and flat roofs.
The essential structural requirement for the mid-depth portion of a transversely loaded straight beam is that it have shear rigidity in planes parallel to the beams longitudinal axis and applied loads, which are usually close to coplanar. Further, this shear rigidity must exist over the full length of the beam. It can be provided by the full width of a monolithic wood beam, by the relatively thin web of a steel I beam, or by the diagonal web bars of an open-web steel joist or wood truss. All provide shear rigidity in the coplane of the beams long axis and transverse loads.
Historically, it has been assumed that this requirement for shear rigidity in the plane of the beam's long axis and applied loads and over the beam's full length was also a requirement for curved beams or arches. All eight of the prior-art U.S. patents relating to non-funicular, moment resisting, open-web arches provide arch connecting means which have shear rigidity in the plane containing the cross section areal centroid curve of the arch and they provide it over the full length of the open-web arch.
Open-web wood arches were disclosed in U.S. Pat. No. 401,870, having webs of radial posts and diagonal braces; U.S. Pat. No. 1,438,452, had double bolted and keyed blocks for web connectors over most of the arch length and diagonal braces near the arch ends; U.S. Pat. No. 1,687,850, used one of two shear web means, double bolted tapered blocks, or a diagonal metal tension strap over radial spacing sleeves and bolts, referred to as a tension and shear member; U.S. Pat. No. 2,390,418, used double bolted blocks spaced uniformly over the full length.
Open-web steel arches were disclosed in U.S. Pat. No. 2,278,797, with diagonal web parts welded to arched channels over the full arch length, with the addition of radial web parts and cross braces near the springing of the arch; U.S. Pat. No. 2,612,854, had diagonal bars or braces welded to arched channels; U.S. Pat. No. 2,666,507, provided spacer plates, which were coplanar with the arches cross section areal centroid curve, welded to steel arches of preferably circular cross section; U.S. Pat. No. 3,530,623, had short, transverse truss braces of hollow square cross section welded to steel channels in and near their curved lengths.
U.S. Pat. No. 3,439,107 disclosed an arch shaped electrical transmission tower made from four support rods of arched shape and transverse spacer rods. It appeard to be an intuitive attempt to provide a funicular shape for both the tower and the individual arches so that they were subjected to primarily longitudinal stresses with small bending stresses. Spacer rods obviously spaced the support rods relative to each other and also shortened their buckling length, though there was no discussion of either funicular shape or buckling.
3. Present Needs
There was no discussion in the reviewed art of open-web arches concerning their buckling characteristics. Some were obviously not self-stable against excessive buckling type rotations and lateral deflections under compressive end loading. The leeward arch is subjected to such compressive buckling loads from wind in the arch span direction. All except U.S. Pat. No. 3,439,107 were intended to have purlins or cladding attached to the outside arch in the direction of arch width. In addition, U.S. Pat. Nos. 2,666,507 and 2,390,418 both provided purlins or stringers, in the arch width direction, through the web volume. Lateral support of the open-web arch by attached structural members, such as purlins, stringers and cladding, can significantly reduce arch buckling rotation about its areal centroid curve and lateral buckling deflections in the arch width direction. That is often a partial purpose.
The recent availability of transparent architectural glazing panels of UV light durable polycarbonate and acrylic plastics in four and six foot widths and continuous lengths, makes possible the continuous glazing from base to ridge of an arch framed structure. It is often desirable that this continuous glazing not be interrupted by horizontal purlins or glazing bars for visual and aesthetic reasons and for minimizing horizontal weather seals. Due to the high coefficients of thermal expansion, these plastic glazing panels are not directly connected to the outside arches, but are free to slide between weather seals. Therefore, they provide little or no lateral support for the arched framing members, which must be self supporting against excessive buckling rotations and deflections over their full length from base to structure ridge. This requirement for open-web arches for small span structures which are self-stable under compressive buckling loads is a recent one, resulting from the availability of large plastic glazing panels.
The open-web arches of the prior-art were not aesthetically pleasing and were intended for agricultural, industrial and commercial uses. Arched shapes are inherently pleasing, but the cluttered nature of numerous across web connectors dominate their appearance. Also, open-web wood arches predated the availability of water durable structural adhesives and were "laminated" with bolts and spikes. The welded structural steel open-web arches would have been hot dipped galvanized, painted or used bare. Consequently, there exists a present need for aesthetically pleasing open-web arches.
Any new open-web arch that provides buckling self-stability and a pleasing appearance must also economically provide for an arch depth which varies over the arch length, as does the distribution of internal bending moments under worst-case wind and/or snow loading. Four of the older prior-art patents did not. Four of the newer did.
There is also an increasing need for greater structural efficiency, i.e. providing structures which are sufficiently strong and stiff, but with less structural material. This requires greater arch depth to resist the load moments, with acceptable deflections and less arch material and greater arch width for buckling self-stability.
Much of the turn-key cost of a residential structure is for construction. There is a need for manufactured arched framing members which can be assembled into an arched structure by one or two lay persons using common hand tools and equipment.